Masteries

Offense:
9

Defense:
1

Utility:
20

I usually play amumu a lot. Here is my common built of amumu. Even though, I usually change the built along with enemy.

The item build is not fix. After buying sunfire cap, look at the enemy champion.

If many of them is physical damage dealer, buy:
-Thornmail
-Frozen Heart
-Upgrade shoes to Mercurt Thread/Boots of Swiftness (others shoes is not recomended)

If many of them is magic damage dealer, buy:
-Banshee' Veil
-Abyssal Scepter
-Upgrade shoes to Mercury Thread

If the games last so long -> buy Guardian Angel

The most important thing is, just add your AP until around 100, not more. So the option items are:
-Abyssal Scepter -> if many caster in enemy team
-Rylai's Crystal Scepter -> if many DPS in enemy team
-Will of the Ancients -> if you want to try it
But just pick 1 of 3 options, not more.